Vincent Tong-Vuong Tran, 25, has been charged with breaching defence legislation after allegedly serving in Ukraine's military between May 20, 2025, and January 14, 2026, without authorisation.
Australian laws limit the work defence personnel can perform with a foreign military, government or company without authorisation.
Tran, who is from Felixstow in Adelaide's east, was charged following an investigation by Australian Federal Police and faced Adelaide Magistrates Court on Tuesday, when he was granted bail.
AFP investigators executed a search warrant at a property in Felixstow on Thursday, seizing electronic devices including a mobile phone and laptop.Â
Forensic examination of the devices allegedly revealed images showing Tran participating in a foreign conflict.
Tran is charged with one count of a restricted individual working for a foreign military organisation or government body, which carries a maximum penalty of 20 years in prison.
He will return to Adelaide Magistrates Court on May 12.
Commenting on the arrest, Ukraine's Ambassador to Australia Vasyl Myroshnychenko told AAP "all Australian engagement with Ukraine's defence must adhere to the requirements of Australian legislation".
There appeared to be a contradiction in Australia strategically supporting Ukraine while charging a person who fought for the country, Flinders University researcher and long-time ADF member Andrew Prevett said.
"This prosecution treats a supporter of a democratic ally as a national security threat, and that kind of defies both logic and our own foreign policy," he told AAP.
Australia has provided more than $1.7 billion worth of assistance to Ukraine since the conflict began, including more than $1.5 billion in military assistance.
Mr Prevett said the ADF also invites personnel who have returned from fighting in Ukraine onto its military bases to speak with Australian troops about their experiences - including drone tactics - and trains members of the Armed Forces of Ukraine under operation Kudu.
"We've got the Australian government largely supporting Ukraine in that war as one of our allies," he said.
"At the same time, we have this reservist who returns getting arrested."
In May 2025, Melbourne man Oscar Jenkins was convicted by a Russian court after allegedly joining the fight in Ukraine and was jailed for 13 years.
Mr Jenkins was captured in occupied eastern Ukraine and taken into Russian custody in December 2024, believed to be the first Australian combatant reportedly captured by Russian forces.
At the time, the Australian government insisted Mr Jenkins was fighting as a member of the Ukrainian defence forces and should be entitled to humane treatment as a prisoner of war.
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